1. Field of the Invention
The present invention generally relates to semiconductor devices, and in particular to a high electron mobility transistor that uses a mixed crystal of a compound semiconductor material.
2. Description of the Related Art
Recently as seen in a portable phone, satellite broadcasting and satellite communication, numerous communication systems using high-frequency waves such as microwaves or millimeter waves have been developed. In these systems, a high-power amplifier is indispensable for the final stage amplifier of the signal-transmitter unit thereof. Therefore, in view of the use of ultra high-frequency band in such systems, a high electron mobility transistor, also called HEMT, is widely used as the high-power output device. It should be noted that a HEMT is a transistor having superior high-frequency characteristics.
Hereinafter the structure and operation of a conventional HEMT used for high frequency and high power application will be described with reference to FIGS. 1-6.
FIG. 1 is a diagram showing the structure of a conventional HEMT. As shown in FIG. 1, a conventional HEMT includes an undoped GaAs layer 2 acting as a channel layer on a semi-insulating substrate 1 of GaAs. On the undoped GaAs layer 2, an undoped Al.sub.x Ga.sub.1-x As layer 4 is grown as a spacer layer, and an n.sup.+ -type Al.sub.x Ga.sub.1-x As layer 5 is grown on the undoped Al.sub.x Ga.sub.1-x As layer 4 as an electron supplying layer. On the n.sup.+ -type Al.sub.x Ga.sub.1-x As layer 5, there are provided an undoped Al.sub.x Ga.sub.1-x As layer 6, an undoped GaAs layer 7, and an n.sup.+ -type GaAs layer 8. Further, a two-dimensional electron gas 3 is formed in the undoped GaAs layer 2 along an interface to the undoped Al.sub.x Ga.sub.1-x As layer 4. The conventional HEMT also includes a gate electrode 9, a source electrode 10, and a drain electrode 11.
As seen above, the HEMT for high-power applications has the undoped layer 6 (which may also be an n.sup.- -type layer) of Al.sub.x Ga.sub.1-x As for increasing the breakdown voltage of the HEMT. Further, a pair of undoped GaAs layers 7 are provided so as to laterally sandwich the gate electrode 9. In the structure of FIG. 1, the Al-content X of the layer 4 or 5, represented by the composition Al.sub.x Ga.sub.1-x As, is desired to have a large value for improving the sheet density of the two-dimensional electron gas 3. however, when the parameter X is too large, the electron density of the two-dimensional electron gas 3 is easily saturated due to the fact that the donor impurity level becomes too deep. Further, the operation of the HEMT tends to become unstable as the HEMT begins to show an optical response. Therefore, the compositional parameter X of the Al.sub.x Ga.sub.1-x As in the layer 4 or layer 5 has been generally set to fall in the range of 0.2.about.0.3.
FIG. 2 is a diagram showing the band structure of the HEMT of FIG. 1 under a thermal equilibrium state. In FIG. 2, the relationship between the valence band Ev, the conduction band Ec, the Fermi level E.sub.F, the ground state energy level Eo of electrons, and the first-excited energy level E.sub.1, of the electrons, is represented.
FIG. 3 is a diagram showing the electron density distribution of the conventional HEMT of FIG. 1 under a thermal equilibrium state, wherein FIG. 3 shows the case in which the HEMT constitutes a normally-on device. FIG. 4 is a diagram showing the band structure of the HEMT under a biased state in which a gate bias voltage is applied in the three-terminal circuit model for causing the HEMT to turn on. Further, FIG. 5 is a diagram showing the electron density distribution of the HEMT in the aforementioned biased state.
Referring to FIG. 2 and FIG. 3, the electron supplying layer 5 is entirely depleted under the condition of thermal equilibrium. When a bias voltage is applied to the electrode 9, on the other hand, an electrically neutral region appears in the layer 5 and grows with an increase of the biased voltage. Thus, as shown in FIG. 5, the electron density of the n.sup.+ -type Al.sub.x Ga.sub.1-x As layer 5 increases with the gate voltage. It should be noted that the drift velocity of the electrons in the electron supplying layer 5 of n.sup.+ -type Al.sub.x Ga.sub.1-x As is lower than that in the channel layer 2 of undoped GaAs. Further, in view of the fact that the electrons in the layer 5 flow to the gate electrode 9 under such a state, the HEMT of FIG. 1 suffers from the problem of drastic decrease of the transconductance g.sub.m, which tends to occur when the gate bias voltage is increased.
FIG. 6 is a diagram showing the relationship between the gate voltage V.sub.g and the transconductance g.sub.m. In FIG. 6, the broken line 60 shows the characteristic of the conventional HEMT, while the solid line 61 shows the characteristic of the HEMT of the present invention to be described later.
Further, there is a HEMT having the electron supplying layer 5 formed of a superlattice structure of n.sup.+ -type GaAs and i-type AlAs. In this prior art HEMT, the aluminum atoms and the silicon atoms, the silicon atoms being doped as donors, are separated spatially from each other so as to minimize the interaction between the aluminum atoms and the silicon atoms. It should be noted that it is this interaction between Al and Si that makes the donor impurity level deep. Thereby the HEMT successfully avoids the problem of saturation of the electron density in the two-dimensional electron gas 3 and the problem of unstability of the HEMT operation caused by the optical response.
FIG. 7 is a diagram showing the band structure of the electron supplying layer having the superlattice structure consisting of n.sup.+ -type GaAs 13 and i-type AlAs 12, wherein the bend of the energy band is omitted. As shown in FIG. 7, the effective energy band gap Eg is defined as the difference between the energy level E.sub.Qe for the ground state of the electrons and the energy level E.sub.Qh for the ground state of the holes. It should be noted that the energy level of the electrons and holes is quantized as a result of formation of the superlattice structure. By choosing a proper thickness for the n.sup.+ -type GaAs layer 13, the energy gap Eg can be set equal to or greater than the gap energy for the case in which the compositional parameter X of the n.sup.+ -type Al.sub.X Ga.sub.1-X As layer 5 is set to about 0.3.
Therefore the electrons in the electron supplying layer having such a superlattice structure are not confined in the quantum well (n.sup.+ -type GaAs layer 13) of the superlattice under the thermal equilibrium state. In FIG. 7, it should be noted that the conduction band Ec of the i-type AlAs layer 12 is for the one at the .GAMMA.-valley.
However, even though the HEMT has such a structure, the electrons in the two-dimensional electron gas 3 are accelerated and flow easily into the electron supplying layer when a large drain current flows. As a result, the drastic decrease of the transconductance g.sub.m is still caused.
FIG. 8 is a diagram of the three-terminal characteristic of the high-power operation of the HEMT of FIG. 7 together with a load line 80. As shown in FIG. 8, the output power decreases due to the decrease of the transconductance g.sub.m in the high-current region 81. Further, it can be seen that the electric power gain, which depends on the mean value of the transconductance g.sub.m, decreases also along the entire load line 80. Further, there is induced a decrease in the drain efficiency and power added efficiency as a result of the decrease of the transconductance g.sub.m at the proximity of a knee voltage. These drawbacks of conventional HEMT cause serious problems particularly when the HEMT is used for high-power applications.